1. Field of the Invention
The present invention relates to a power supply device and an image forming apparatus including the power supply device.
2. Description of the Related Art
Conventionally, a high voltage AC power supply is required for performing a charging process such as an AC charging process. In order to control a transformer drive circuit with the high voltage AC power supply, a DC voltage is generated and fed back with a transformer output waveform signal via, for example, a rectifier and a low pass filter. However, the transformer drive circuit of an AC charger in a conventional image forming apparatus includes, for example, a half wave rectifier and a low pass filter for obtaining a feedback DC voltage used in controlling the transformer drive circuit. With such transformer drive circuit, the AC frequency for driving the transformer is low (approximately 1 kHz), it is necessary to delay the time constant of, for example, the half wave rectifier and the low pass filter. As a result, the speed for controlling the transformer drive circuit is delayed for approximately several tens of kHz. Therefore, the transformer drive circuit is unable to track (follow) a load that changes faster than the control speed of the transformer drive circuit.
FIG. 21 is a block diagram for describing an AC voltage generation part of a high voltage AC power supply device 100 for controlling an AC transformer drive according to a related art example. In FIG. 21, the high voltage AC power supply device 100 includes an IC (integrated circuit) 68. A PWM (pulse width modulation) filter 50, a differential integrator 51, a sinusoidal wave generation part 52, a triangular wave generation part 53, a comparator 54, and a switching driver part 55 constitute the IC 68. The PWM filter 50 generates a set voltage 57 in accordance with an AC_PWM signal 56 input to the PWM filter 50. The difference between the set voltage 57 and the output amplification feedback signal 67 is accumulated by the differential integrator 51.
An integrated value and a frequency setting clock AC_CLK 59 are input to the sinusoidal wave generation part 52. The sinusoidal wave generation part 52 controls amplification (frequency) in accordance with the integrated value and generates an AC signal (sinusoidal wave) 60 having a frequency controlled in accordance with the frequency setting clock AC_CLK 59. The generated AC signal 60 is also referred to as a first sinusoidal wave signal. The first sinusoidal wave signal 60 generated by the sinusoidal wave generation part 52 and a triangular wave (triangular wave signal) 69 generated by the triangular wave generation part 53 are input to the comparator 54. Based on the input, the comparator 54 generates a PWM signal 61. The comparator 54 may be, for example, a typical differential comparator. The switching drive part 55 amplifies the PWM signal 61 and outputs an amplified PWM signal 62 outside of the IC 68. The LPF (Low Pass Filter) 63 converts the amplified PWM signal 62 to a second sinusoidal wave signal having an amplified voltage compared to that of the first sinusoidal wave signal. A high voltage transformer 64 is driven by the second sinusoidal wave signal input thereto. The high voltage transformer further amplifies the second sinusoidal wave signal and outputs an AC voltage 65 having a frequency that is set in accordance with the frequency setting clock AC_CLK 59.
FIG. 20 is a schematic diagram illustrating a feedback system according to a related art example. The feedback system of FIG. 20 does not directly monitor output. More specifically, the feedback system of FIG. 20 monitors output by using a monitor coil (winding) that is provided separate from the input and output. The control speed of the feedback system of FIG. 20 is several tens of Hz due to the sinusoidal wave being lowered to DC level by the half wave rectifier 69.
As a related art example, Japanese Laid-Open Patent Publication No. 2009-122564 discloses an AC high voltage power supply device provided for the purpose of reducing size and reducing power consumption. In the disclosed AC high voltage power supply device, an input signal or an output signal of a transformer is used as a monitor signal. According to the monitor signal, feedback control is performed and a signal is input to a comparator so that a peak level of an output signal of the transformer becomes a predetermined peak level.
However, with the high voltage AC power supply device of the related art example illustrated in FIG. 21, the output signal or the input signal of the high voltage transformer 64 is rectified by the rectifier 66 and fed back as an output amplitude feedback signal 67. The frequency of an AC voltage output used in a charging process by an image forming apparatus is approximately 1 kHz. In order to change the AC voltage output to a DC voltage by rectifying the AC voltage output and feeding back the rectified signal as the output amplitude feedback signal 67, the cutoff frequency of the rectifier 66 becomes low (several tens of Hz). Thus, the speed of the entire control system of the high voltage AC power supply device 100 becomes low in correspondence with the cutoff frequency. This leads to a problem in which the control of the high voltage AC power supply device 100 cannot follow (trace) the changes of a load.
Further, the high voltage power supply device disclosed in Japanese Laid-Open Patent Publication No. 2009-122564 is also unable to follow (trace) the changes of a load.